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Relation of THD with swing and linearity

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simplsoft

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Hello everyone

I want to know the relation of THD with the output swing. I am working on OTA. I have a THD of -40dB which is bad. Now I have read on some search engine that:

1. To improve THD we have to check the swing.

2. But in different research articles its mentioned that the improvement of THD can be done by improving the Gm linearity. They have shown the graphs between Gm vs Vid for linearity.

Which point is right to improve linearity. Do I have to check the swing?

Thanks
 

Hi,

The linearity is determined in general by the last stage in the signal chain. If the last stage is compressing, the THD will worse. This is for voltage outputs, i.e. Opamps.

The other linearity which you are talking is about handling as large input swing as possible. With large input swing, the input gm can enter into non-linear regions. Thus, to increase linearity, one needs to linearize the gm over as large as input range as possibl.e
 

In a transistor, gm varies with the transistor current so large voltage (current) swings will cause non-linearity and distortion due to this change in gm.
The usual way to reduce the effects of this non-linearity is with negative feedback.
One way to do this is with an unbypassed resistor in series with the emitter/source of the transistor.
This of course, reduces the stage gain, so there's a tradeoff between gain and distortion.
You thus may have to add another stage to achieve the desired amplifier gain.

Below is the LTspice simulation of a single BJT stage showing the difference is output distortion and gain both with and without the 100Ω emitter resistor being AC bypassed by switch S1.

The signal is very distorted when the resistor is bypassed.

Notice how the input signal had to be increased to give the same approximate output voltage due to the reduction in gain when the emitter resistor is not bypassed.

Capture.PNG
 

Thankyou for your reply. It means we dont need large input signal swing because it causes the input gm to enter the non-linear region. ? M I right?

Can you plz help me how to chk the input and output swing? I am using cadence software.

Thankx
 

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